The present invention relates to measurement and dilution techniques for analysis of gaseous constituents.
A gas diluting and testing apparatus is used to analyze, among other things, vehicular exhaust. The apparatus uses a mixing system to dilute the exhaust gases so that the moisture content of the gases is sufficiently reduced in order to minimize errors due to condensation. One example of an existing mixing system is the Constant Volume Sampler (CVS).
The CVS has been used for over twenty-five years to sample the emissions from automobiles. It is the device used for all automotive emissions tests that are the basis for the certification that vehicles sold in the United States are compliant with the Clean Air Act.
As vehicles become cleaner, their emissions become more difficult to measure accurately. In particular, because the basic operation of the CVS involves diluting the vehicle""s exhaust with air that may also contain small amounts of the pollutants to be measured, it becomes important to be able to accurately separate out the contributions from this source and the contribution from the vehicle under test.
Historically, this ambient contribution has been corrected for by taking a sample of the diluent air at the same time that a sample of the diluted vehicle exhaust is taken. This measurement, with some modifications to account for the different conditions of sampling, is subtracted from the measurement made of the diluted exhaust. However, this method makes some assumptions about the stability of the background pollutants and about the combustion process. Because these assumptions are not completely true, the errors caused by these assumptions have become significant as vehicles have become cleaner.
Referring to FIG. 1, a conventional CVS is generally indicated at 10. CVS 10 has a mixing portion 12, a gaseous inlet 14, a dilution inlet 16, and a mixture outlet 18. A dilution sample line 20, a needle valve 22 and a pump 24 cooperate to a bag 26 with dilution gas.
Raw exhaust from the vehicle under test enters gaseous inlet 14 and is mixed with dilution air. The mixed gases are drawn through a main venturi 30 by a blower 32. Main venturi 30 is sonic, or choked, and meters and measures the flow of the combined gases. A mixture sample line 34 connects to mixture outlet 18 through a smaller, sample venturi 36 also operated in sonic or choked condition. A pump 38 cooperates with sample venturi 36 to fill a sample bag 40 with the mixed gases for later analysis.
Because there are measurable amounts of the pollutants in the dilution gas used to dilute the exhaust gases, the dilution gas sample in bag 26, which is collected through pump 24 and needle valve 22 at a nominally constant flow rate, is analyzed. Typically, the pollutant concentration in the dilution bag 26 and in the sample bag 40 are used to determine pollutant emissions according to the following known formula:
xe2x80x83gramsC=densityC*([C]samxe2x80x94bagxe2x88x92[C]ambxe2x80x94bag(1xe2x88x921/DF))*Vcvs
wherein gramsC is the gaseous constituent content in the test gas, densityc is the density of the gaseous constituent, [C]samxe2x80x94bag is the concentration of the gaseous constituent in the gaseous mixture sample, [C]ambxe2x80x94bag is the concentration of the gaseous constituent in the dilution gas sample, DF is the theoretical dilution factor (determined as indicated below), and VCVS is the total volume of the gaseous mixture drawn through the system.
The dilution factor, DF, is the ratio of the total volume of gases taken through the CVS to the volume of exhaust gases from the test vehicle. In the conventional CVS, the dilution factor DF is specified by the following theoretical formula:   DF  =            x              x        +                  y          2                +                  3.76          ⁢                      (                          x              +                              y                4                            -                              z                2                                      )                                                        [                      CO            2                    ]                sam_bag            +                        [          CO          ]                sam_bag            +                        [          HC          ]                sam_bag            
wherein x, y, and z come from the composition of the fuel, which is considered to have the chemical formula CxHyO. Further, in the above formula [CO2]samxe2x80x94bag is the concentration of carbon dioxide in the gaseous mixture sample, [CO]samxe2x80x94bag is the concentration of carbon monoxide in the gaseous mixture sample, and [HC]samxe2x80x94bag is the concentration of hydrocarbons in the gaseous mixture sample.
Although conventional mixing systems of the CVS type have been used in many applications which have been commercially successful, it is to be appreciated that as the amounts of pollutants in vehicular exhaust decrease, the assumptions made for the conventional CVS have an increasing effect on the accuracy of the testing. Because the contribution of the dilution gas to the overall pollutant content of the gaseous mixture sample is becoming more and more significant in the calculation of exhaust gas pollutant content, there is a need for an improved mixing system that overcomes the problems and limitations of the prior art.
It is, therefore, an object of the present invention to provide a mixing system utilizing proportional dilution gas sampling, and thereby increasing the accuracy of the testing program.
It is a further object of the present invention to provide proportional dilution gas sampling methods.
In carrying out the above objects and other objects and features of the present invention, a mixing system for diluting gases is provided. The mixing system comprises a mixing portion, gaseous inlet, a dilution inlet, and a mixture outlet. The gaseous inlet is in flow communication with the mixing portion and receives the test gases. The dilution inlet is in flow communication with the mixing portion and receives the dilution gas. The mixture outlet is also in flow communication with the mixing portion.
The mixing system further comprises a flow meter in flow communication with the dilution inlet, a dilution sample line in flow communication with the dilution inlet, and a dilution sample flow controller along the dilution sample line. The flow meter has an output indicative of a dilution gas flow rate. The dilution inlet collects samples of the dilution gas; and, the dilution sample flow controller is operable to vary a dilution sample flow rate through the dilution sample line.
Control logic receives the flow meter output, and processes the flow meter output. The control logic operates the dilution sample flow controller to vary the dilution sample flow rate in response to variations in the dilution gas flow rate. The dilution sample flow rate is varied such that the dilution sample flow rate tracks a value which is substantially proportional to the dilution gas flow rate.
Further, in carrying out the present invention, a method for determining a gaseous constituent content in a test gas using a mixing system is provided. The method comprises mixing a dilution gas with the test gas to produce a gaseous mixture. The dilution gas has a dilution gas flow rate. The gaseous mixture has a gaseous mixture flow rate. The method further comprises monitoring the dilution gas flow rate, and sampling the dilution gas at a dilution sampling rate that is substantially proportional to the dilution gas flow rate to obtain a dilution gas sample. The gaseous mixture is sampled to obtain a gaseous mixture sample. A gaseous constituent content in the gaseous mixture sample is determined; a gaseous constituent content in the dilution gas sample is also determined. The gaseous constituent content in the test gas is determined based on the gaseous constituent content in the gaseous mixture sample, and the gaseous constituent content in the dilution gas sample.
Still further, in carrying out the present invention, a method for operating a mixing system is provided. The method comprises mixing a diluting gas with a test gas to produce a gaseous mixture, monitoring the dilution gas flow rate, sampling the dilution gas at a dilution sampling rate that is substantially proportional to the dilution gas flow rate to obtain a dilution gas sample, and sampling the gaseous mixture to obtain a gaseous mixture sample.
The advantages associated with embodiments of the present invention are numerous. For example, a mixing system made in accordance with the present invention functions as a Constant Volume Sampler (CVS) with proportional dilution gas sampling. The proportional dilution gas sampling allows true calculation of gaseous constituent content in the dilution gas, avoiding the need for assumptions made in a conventional CVS that may result in inaccurate test results. Further, because the CVS of the present invention with proportional dilution gas sampling avoids assumptions made with a conventional CVS, the accuracy of the vehicle emissions measurement is increased. Additionally, embodiments of the present invention offer a cost savings for the filtering and conditioning of the dilution gas, and performance improvements and the lowering of sampling back-pressure on the vehicle exhaust pipe during vehicular emissions testing.
The above objects and other objects, features, and advantages of the present invention are readily apparent from the following detailed description of the best mode for carrying out the invention when taken in connection with the accompanying drawings.